American Express is already experimenting with its next-gen mobile payment system Serve, testing the waters for the future of how we'll buy things. Now it's taken an interesting step that taps into current social gaming mores: It's launched a rewards scheme that doesn't give users of its pre-paid Serve cards airmiles or discounts in stores...instead it offers virtual rewards that can be spent in Zynga's wildly popular game FarmVille. The Zynga Serve Rewards system will soon also plug into other Zynga games, and crucially it'll also operate the other way around...with Zynga planning on letting the games act as marketing tools themselves as brands will be able to pop offers into games that lead to benefits that can be redeemed using Serve-powered credit cards.